Process for preparing a catalyst for producing methacrylic acid

ABSTRACT

When a catalyst for producing methacrylic acid is prepared by shaping a catalyst component represented by the general formula P a  Mo b  V c  X d  Y e  Z f  O g  wherein P, Mo, V, X, Y, Z, a, b, c, d, e, f and g are as defined in the specification, a catalyst capable of giving methacrylic acid in high yield is provided by adding one or more organic high-molecular weight compounds with an average particle size of 0.01-10 μm, and carrying out heat treatment before use as a catalyst.

TECHNICAL FIELD

The present invention relates to a process for preparing a catalyst usedfor producing methacrylic acid by vapor phase catalytic oxidation ofmethacrolein.

BACKGROUND ART

There have been made a large number of proposals on processes andcatalysts for producing methacrylic acid by vapor phase catalyticoxidation of methacrolein. In one of these proposals, there has beendisclosed a process using an organic substance such as a cellulose, apolyvinyl alcohol or the like in preparation of a catalyst, for thepurpose of controlling pores (the specification of British PatentLaid-Open Application No. 2037604). As an invention for controllingpores, there is the invention disclosed in Japanese Patent UnexaminedPublication No. 60-239439. These catalysts, however, have defects suchas unsatisfactory reaction results, a large decrease of catalyticactivity with the lapse of time, and a troublesome after-treatment, andthere is a desire to seek further improvement in them for theiremployment as industrial catalysts.

DISCLOSURE OF THE INVENTION

The present invention is intended to provide a novel process forpreparing a catalyst for advantageous production of methacrylic acidfrom methacrolein.

The present inventors earnestly investigated in order to improveconventional catalyst preparation processes, and consequently found anovel process for preparing a catalyst which gives mothacrylic acid in ahigher yield than does that prepared by a conventional process. Thepresent invention is a process for preparing a catalyst for producingmethacrylic acid by vapor phase catalytic oxidation of methacroiein withmolecular oxygen which catalyst is obtained by adding an organichigh-molecular weight compound with an average particle size of 0.01 μmto 10 μm to a catalyst component having a composition represented by theconoral formula:

    P.sub.a Mo.sub.b V.sub.c X.sub.d Y.sub.e Z.sub.f O.sub.g

(wherein P, Mo, V and O denote phosphorus, molybdenum, vanadium andoxygen, respectively; X denotes at least one element selected from thegroup consisting of arsenic, antimony, bismuth, germanium, zirconium,tellurium, silver and boron; Y denotes at least one element selectedfrom the group consisting of iron, copper, zinc, chromium, magnesium,tantalum, manganese, barium, gallium, cerium and lanthanum; Z denotes atleast one element selected from the group consisting of potassium,rubidium, cesium and thallium; a, b, c, d, e and f denotes atomic ratiovalues for the individual elements: in the case of b being 12, a=0.5 to3, c=0.01 to 3, d=0 to 3, e=0 to 3, f=0.01 to 3; and g is a number ofoxygen atoms which is necessary for giving the above valences of theindividual constituents) to obtain a mixture, and molding the resultingmixture, followed by heat treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, it is important that at the time of themolding of the catalyst, the organic high-molecular weight compound withan average particle size in a specified range of 0.01 μm to 10 μm hasbeen previously added to the catalyst components. When the averageparticle size of the organic high-molecular weight compound is less than0.01 μm, there are caused problems in handling, for example, secondaryaggregation of particles of the organic high-molecular weight compoundused occurs, and dust tends to be raised during handling of the organichigh-molecular weight compound. When the average particle size is morethan 10 μm, the proportion of pores preferable for the reaction in thecatalyst obtained by the heat treatment after the molding is decreased,so that the catalytic capability is deteriorated.

Although the effect of the addition of the organic high-molecular weightcompound with the above specified average particle size on theimprovement of the catalytic capability is not exactly clear, it can bespeculated from the increase of pores with a diameter of approximately100-10,000 Å in the catalyst by the heat treatment (200°-500° C.)carried out after the molding into the catalyst that a pore structureideal for the oxidation reaction for production of methacrylic acid frommethacrolein is formed in the catalyst by the addition of the organichigh-molecular weight compound.

The particle size of the organic high-molecular weight compound wasmeasured under a scanning electron microscope, and the diameter of poresin the catalyst was measured by a mercury penetrating method.

In the present invention, although the organic high-molecular weightcompound incorporated into the catalyst is not critical so long as ithas an average article size in the above range, those which are easilyremovable by decomposition by heating or combustion, for example,polymers of methyl, ethyl, propyl, butyl or isobutyl esters of(meth)acrylic acid, and polymers of styrene, α-methylstyrene or thelike, are preferable. In particular, those which decompose into amonomer at a relatively low temperature to vaporize and evaporate, forexample, polymethyl methacrylates and polystyrenes, are preferable.These compounds may be used singly or as a mixture of two or morethereof.

The adding amount of the organic high-molecular weight compound issuitably 0.1 to 30% by weight based on the weight of the catalyst. Whenthe adding amount is too small, the addition has no effect. When theadding amount is too large, the mechanical strength of the catalystafter the heat treatment is lowered. Therefore, both of such amounts arenot desirable.

A process for producing a catalyst having the composition employed inthe present invention is not critical. Various heretofore well-knownmethods such as an evaporation-to-dryness method, precipitation method,oxide-mixing method, etc. can be used unless a markedly unevendistribution of the constituents occurs. The organic high-molecularweight compound is added in wet or dry state to a powdery catalystcomponent obtained by such a method, after which the resulting mixtureis molded into a catalyst of a desired shape by supporting the same on acarrier or tableting the same. The organic high-molecular weightcompound can be removed by heat-treating the catalyst obtained by themolding, at 200°-500° C., preferably 300°-450° C.

As starting materials for the catalyst component having a compositionrepresented by the above general formula P_(a) Mo_(b) V_(c) X_(d) Y_(e)Z_(f) O_(x), oxides, nitrates, carbonates, ammonium salts, halides, etc.of the individual elements can be used in combination. For example, as amaterial for molybdenum, there can be used ammonium paramolybdate,molybdenum trioxide, molybdenum chloride, etc. As a material forvanadium, there can be used ammnonium metavanadate, vanadium pentaoxide,vanadium chloride, etc.

Although the catalyst used in the process of the present invention maybe free from a carrier, it may be used after being supported on an inertcarrier of silica, alumina, silica-alumina, magnesia, titania, siliconcarbide or the like, or after being diluted with the inert carrier.

When the catalyst obtained according to the present invention isutilized, the concentration of methacrolein in a starting gas can bevaried in a wide range, though it is suitably 1 to 20% by volume,particularly preferably 3 to 10% by volume. The starting methacroleinmay contain a small amount of impurities such as water, lower saturatedaldehydes, etc., and these impurities have no substantial influence onthe reaction.

Although employment of air as an oxygen source for carrying out thecatalytic oxidation is economical, air enriched with pure oxygen mayalso be used if necessary. The oxygen concentration in the starting gasis defined as the molar ratio of oxygen to methacrolein, and the valueof the ratio is preferably 0.3 to 4, in particular, 0.4 to 2.5. Thestarting gas may be diluted with an inert gas such as nitrogen, watervapor, or carbon dioxide gas.

The reaction pressure is preferably atomospheric pressure to severalatmospheres. Although the reaction temperature can be chosen in therange of 230°-450° C. it is particularly preferably 250°-400° C. Thereaction can be carried out either on a fixed bed or on a fluidized bed.

EXAMPLES

The process for preparing the catalyst according to the presentinvention and examples of reaction using said catalyst are concretelyexplained below.

In the examples and comparative examples, the conversion of methacroleinand the selectivity of methacrylic acid produced are defined by thefollowing: ##EQU1##

Parts in the following examples and comparative examples are by weight,and the analysis was carried out by a gas chromatography.

Example 1

In 300 parts of pure water were dissolved 100 parts of ammoniumparamolybdate, 1.66 parts of ammonium metavanadate and 4.77 parts ofpotassium nitrate. A solution of 8.16 parts of 85% phosphoric acid in 10parts of pure water was added thereto, and then 2.75 parts of antimonytrioxide was added, after which the resulting mixture was heated to 95°C. with stirring. Subsequently, a solution of 1.14 parts of coppernitrate in 30 parts of pure water was added, and the resulting mixturewas evaporated to dryness with heating and stirring.

The solid thus obtained was dried at 130° C. for 16 hours, and then apolymethyl methacrylate (hereinafter abbreviated as PMMA) with anaverage particle size of 0.15 μm was added in an amount of 3 parts per100 parts of the dried solid and mixed therewith, after which theresulting mixture was molded under pressure molded and heat-treated at380° C. for 5 hours while introducing air. This product was used as acatalyst.

The composition determined for elements other than oxygen (hereinafterthe same applied) of the catalyst obtained was P₁.5 Mo₁₂ V₀.3 Sb₀.4Cu₀.1 K₁.

This catalyst was packed into a reaction tube, and a mixed gasconsisting of 5% of methacrolein, 10% of oxygen, 30% of water vapor and55% of nitrogen (% by volume) was introduced thereinto at a reactiontemperature of 270° C. for a contact time of 3.6 seconds. The productwas collected and then analyzed by a gas chromatography to find that theconversion of methacrolein was 80.3% and the selectivity of methacrylicacid 81.4%.

Comparative Example 1

When a catalyst for comparison P₁.5 Mo₁₂ V₀.3 Sb₀.4 Cu₀.1 K₁ wasprepared according to Example 1, except that PMMA was not added at thetime of the molding under pressure, and the reaction was carried outunder the same reaction conditions as in Example 1 by the use of thiscatalyst, the conversion of methacrolein was 80.6% and the selectivityof methacrylic acid 79.7%.

Comparative Example 2

When a catalyst for comparison P₁.5 Mo₁₂ V₀.3 Sb₀.4 Cu₀.1 K₁ wasprepared according to Example 1, except that the average particle sizeof PMMA added at the time of molding under pressure was 20 μm, and thereaction was carried out under the same conditions as in Example 1 bythe use of this catalyst, the conversion of methacrolein was 80.4% andthe selectivity of methacrylic acid 79.3%. These results are equal to orsomewhat inferior to those obtained when PMMA was not added at all,indicating that the average particle size of a substance added is ofimportant significance.

Example 2

With 800 parts of pure water were mixed 100 parts of molybdenumtrioxide, 2.63 parts of vanadium pentaoxide and 6.67 parts of 85%phosphoric acid. The resulting mixture was stirred with heating underreflux for 3 hours, after which 0.92 parts of copper oxide was added,and heating with stirring under reflux was conducted again for 2 hours.The resulting slurry was cooled to 50° C. and a solution of 8.98 partsof cesium hydrogencarbonate in 30 parts of pure water was added and thenstirred for 15 minutes. Then, a solution of 10 parts of ammonium nitratein 30 parts of pure water was added, and the resulting mixture wasevaporated to dryness with heating at 100° C. and stirring.

The solid obtained was dried at 130° C. for 16 hours, after which apolystyrene with an average particle size of 5 μm was added in an amountof 3 parts per 100 parts of the dried solid and mixed therewith, and theresulting mixture was molded under pressure and heat-treated at 380° C.for 3 hours while introducing air. This product was used as a catalyst.

The composition of the catalyst was P₁ Mo₁₂ V₀.5 Cu₀.2 Cs₀.8.

When the reaction was carried out by the use of this catalyst under thesame reaction conditions as in Example 1 except for changing thereaction temperature to 285° C., the conversion of methacrolein was84.9% and the selectivity of methacrylic acid 85.7%.

Comparative Example 3

When a catalyst for comparison P₁ Mo₁₂ V₀.5 Cu₀.2 Cs₀.8 was preparedaccording to Example 2, except that no polystyrene was added at the timeof the molding under pressure, and the reaction was carried out underthe same reaction conditions as in Example 2 by the use of thiscatalyst, the conversion of methacrolein was 85.3% and the selectivityof methacrylic acid 84.2%.

Comparative Example 4

When a catalyst for comparison P₁ Mo₁₂ V₀.5 Cu₀.2 Cs₀.8 was preparedaccording to Example 2, except that the average particle size of apolystyrene added at the time of the molding under pressure was 50 μm,and the reaction was carried out under the same reaction conditions asin Example 2 by the use of this catalyst, the conversion of methacroleinwas 85.0% and the selectivity of methacrylic acid 83.8%.

Examples 3 to 11

The catalysts listed in Table 1 were prepared according to Example 1,and the reaction was carried out under the same conditions as in Example1 except for the reaction temperature to obtain the results shown inTable 1.

COMPARATIVE EXAMPLES 5 TO 13

The catalysts for comparison listed in Table 1 were prepared accordingto Examples 3 to 11, except that no organic high-molecular weightcompound was added at the time of the molding under pressure, and thereaction was carried out under the same conditions as in Examples 3 to11 to obtain the results shown in Table 1.

Comparative Examples 14 TO 22

The catalysts for comparison listed in Table 1 were prepared accordingto Examples 3 to 11, except that as organic high-molecular weightcompound added at the time of the molding under pressure, one or morecompounds having an average particle size of 10 μm or more were used,and the reaction was carried out under the same conditions as inExamples 3 to 11 to obtain the results shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                 Organic high-                                                                 molecular weight                                                              compound added    Conversion                                                                           Selectivity                                          [average particle                                                                        Reaction                                                                             of     of                                                   size (μm)/adding                                                                      temperature                                                                          methacrolein                                                                         methacrylic acid        Composition of catalyst      amount (wt %)]                                                                           (°C.)                                                                         (%)    (%)                     __________________________________________________________________________    Example 3                                                                            P.sub.1 Mo.sub.12 V.sub.0.5 Sb.sub.0.7 Bi.sub.0.2 Zn.sub.0.2                  Ge.sub.0.2 La.sub.0.3 K.sub.1                                                                       PMMA [0.15/5]                                                                            270    89.9   88.7                    Comparative                                                                          "                     No addition                                                                              "      89.6   87.5                    Example 5                                                                     Comparative                                                                          "                     PMMA [100/5]                                                                             "      89.4   87.0                    Example 14                                                                    Example 4                                                                            P.sub.1.5 Mo.sub.12 V.sub.0.8 Te.sub.0.2 Cu.sub.0.1 Mg.sub.0.3                Fe.sub.0.2 Rb.sub.1   PMMA [0.25/3]                                                                            290    87.5   88.5                    Comparative                                                                          "                     No addition                                                                              "      87.3   87.6                    Example 6                                                                     Comparative                                                                          "                     PMMA [20/3]                                                                              "      87.2   86.1                    Example 15                                                                    Example 5                                                                            P.sub.1.3 Mo.sub.12 V.sub.0.4 Sb.sub.0.8 Bi.sub.0.2 Cu.sub.0.1                Ba.sub.0.2 Tl.sub.0.8 PMMA [0.25/4]                                                                            290    87.1   88.5                    Comparative                                                                          "                     No addition                                                                              "      86.6   87.6                    Example 7                                                                     Comparative                                                                          "                     PMMA [50/4]                                                                              "      86.2   87.1                    Example 16                                                                    Example 6                                                                            P.sub.1.4 Mo.sub.12 V.sub.0.4 Ge.sub.0.2 B.sub.0.2 Cu.sub.0.1                 Fe.sub.0.3 K.sub.0.3 Rb.sub.0.8                                                                     PMMA [0.40/2]                                                                            290    88.0   87.5                    Comparative                                                                          "                     No addition                                                                              "      88.2   86.0                    Example 8                                                                     Comparative                                                                          "                     PMMA [100/2]                                                                             "      88.0   85.7                    Example 17                                                                    Example 7                                                                            P.sub.1.2 Mo.sub.12 V.sub.0.5 Ag.sub.0.1 Ta.sub.0.2 Cu.sub.0.2                Cs.sub.1              PMMA [0.80/3]                                                                            290    82.1   87.0                    Comparative                                                                          "                     No addition                                                                              "      83.2   85.2                    Example 9                                                                     Comparative                                                                          "                     PMMA [20/3]                                                                              "      82.6   84.3                    Example 18                                                                    Example 8                                                                            P.sub.2 Mo.sub.12 V.sub.0.5 Zr.sub.0.4 Cu.sub.0.2 Cr.sub.0.5                  Tl.sub.0.8            PMMA [2.0/3]                                                                             290    87.8   86.7                    Comparative                                                                          "                     No addition                                                                              "      87.8   85.5                    Example 10                                                                    Comparative                                                                          "                     PMMA [50/3]                                                                              "      87.5   85.0                    Example 19                                                                    Example 9                                                                            P.sub.1.5 Mo.sub.12 V.sub.0.3 Ag.sub.0.1 Ge.sub.0.3 Cu.sub.0.2                Ba.sub.0.2 Mn.sub.0.05 Cs.sub.1                                                                     Polystyrene [2.0/5]                                                                      290    86.6   88.0                    Comparative                                                                          "                     No addition                                                                              "      86.3   87.0                    Example 11                                                                    Comparative                                                                          "                     PMMA [100/5]                                                                             "      86.0   86.2                    Example 20                                                                    Example 10                                                                           P.sub.1.5 Mo.sub.12 V.sub.0.5 As.sub.0.2 Cu.sub.0.2 K.sub.0.7                 Cs.sub.0.2            PMMA [2.0/3] +                                                                           310    82.9   86.9                                                 Polystyrene [5.0/2]                              Comparative                                                                          "                     No addition                                                                              "      83.0   85.7                    Example 12                                                                    Comparative                                                                          "                     PMMA [100/3] +                                                                           "      82.5   85.3                    Example 21                   Polystyrene [50/2]                               Example 11                                                                           P.sub.1.5 Mo.sub.12 V.sub.0.7 Sb.sub.0.7 Ga.sub.0.2 B.sub.0.2                 Cu.sub.0.2 Zn.sub.0.3 Ge.sub.0.1 K.sub.1                                                            Poly(isobutyl                                                                            290    89.2   87.4                                                 methacrylate) [5.0/3]                            Comparative                                                                          "                     No addition                                                                              "      88.8   86.6                    Example 13                                                                    Comparative                                                                          "                     Poly(isobutyl                                                                            "      88.3   86.0                    Example 22                   methacrylate) [100/3]                            __________________________________________________________________________

INDUSTRIAL APPLICABILITY

A catalyst prepared by the process of the present invention has a porestructure preferable for vapor phase catalytic oxidation reaction ofmethacrolein, and has the effect of improving the selectivity ofmethacrylic acid.

We claim:
 1. A process for preparing a catalyst for producingmethacrylic acid by vapor phase catalytic oxidation of methacrolein,comprising adding one or more organic high-molecular weight compoundsselected from the group consisting of polymethyl methacrylates andpolystyrenes, the compounds having an average particle size of 0.02 μmto 10 μm, to a catalyst component having a composition represented bythe general formula:

    P.sub.a Mo.sub.b V.sub.c X.sub.d Y.sub.c Z.sub.f O.sub.g

(wherein P, Mo, V and O denote phosphorus, molybdenum, vanadium andoxygen, respectively; X denotes at least one element selected from thegroup consisting of arsenic, antimony, bismuth, germanium, zirconium,tellurium, silver and boron; Y denotes at least one element selectedfrom the group consisting of iron, copper, zinc, chromium, magnesium,tantalum, manganese, barium, gallium, cerium and lanthanum; Z denotes atleast one element selected from the group consisting of potassium,rubidium, cesium and thallium; a, b, c, d, e and f note atomic ratiovalues for the individual elements: in the case of b being 12, a=0.5 to3, c=0.01 to 3, d=0. to 3, e=0 to 3, f=0.01 to 3; and g is a number ofoxygen atoms which is necessary for giving the above valences of theindividual constituents) to obtain a mixture, and molding the resultingmixture, followed by a heat treatment to remove the one or more organichigh molecular weight compounds.